Polyolefins, including polyethylene, are important items of commerce, being used for many different applications. One of these applications is blow molding, which is particularly useful for making large hollow items such as bottles, drums and tanks from thermoplastics. Blow molding is typically divided into three types of operations: extrusion blow molding, injection blow molding and stretch blow molding. In a typical extrusion blow molding operation, a tube of molten polymer is extruded, and then the open end of the tube is sealed. The sealed tube, called a “parison”, is enclosed by a (split) mold, and then the interior of the tube is subjected to gas pressure (“blown”) so the it is pressed against the surface of the cool mold, thereby forming a closed shape. The tube end of the shaped article is then cut off and trimmed, and a hollow shaped article results. This method is particularly useful for producing large hollow articles, since the pressures required are low.
In the formation and working of the parison, it is preferred that the molten thermoplastic has certain viscoelastic properties. For formation of the parison by extrusion, it is preferred that the thermoplastic not have too high a melt viscosity at relatively high shear rates. This limits the amount of very high molecular weight polymer present; otherwise, the extrusion step may be too difficult. On the other hand, one prefers that the low shear viscosity (sometimes also called the melt strength) of the polymer be high to avoid “sag.” During and after parison formation, the parison is usually suspended for a short period from the extrusion die, and one does not want the parison to flow or deform significantly (“sag”) before the actual molding step. High melt strength is often imparted by high molecular weight polymer. Therefore, polymers useful for blow molding often have a relatively small amount of a high molecular weight fraction present to impart good melt strength without making the high shear viscosity too high.
U.S. Pat. Nos. 6,214,761 and 6,297,338 describe the use of polymerization catalyst systems containing at least one iron complex of a bisimine of 2,6-diacylpyridine or a bisimine of a 2,6-pyridinedicarboxaldehyde and at least one other polymerization catalyst, which also may be an iron complex of a bisimine of 2,6-diacylpyridine or a bisimine of a 2,6-pyridinedicarboxaldehyde. No mention is made of the specific composition described herein or of its use to make polymers particularly useful in blow molding.
World Patent Application 01/15899 describes the use of polyethylenes made with iron complexes of a bisimine of 2,6-diacylpyridine or a bisimine of a 2,6-pyridinedicarboxaldehyde in blow molding. No mention is made of using a polyethylene prepared from a mixture of two such polymerization catalysts.